Studies into Ash Deposition Propensities in the Vicinity of Economizer and Air Preheater of Coal-Fired Power Utility

Coal is still massively utilized as a main fuel in several countries for power plants or steel industry. However, global coal reserves currently are dominated by low quality coal, having calorific values lower than 25.52 MJ/kg and containing high alkali, sulfur, and ash content, that can promote ash deposition problems. The objective of this study is to investigate coal selection for steam power plants focusing on ash deposition risks on the vicinity of economizer and air preheater, approximately at 400°C. This study has two evaluations covering the original coals and coal with the highest risk of ash deposition using additive. The original coals in this study are obtained from Kalimantan, Indonesia, which are coded MH, BE, ES, SG, and AD. According to the result of characteristic test, MH, BE, and ES contain a high Na₂O content, SG contains a high sulfur, while AD has a high MgO content. The first evaluation show that ES has the highest ash deposit risk, reflected by drop tube furnace (DTF) combustion test result with high ash deposition rate (0.1424 g/h) and more melting spots found on the probe surface. Ash particle observations using x-ray diffraction (XRD) and scanning electron microscope–energy dispersive spectroscopy (SEM-EDS) also show the presence of Na-based minerals and elements in the ash particles. In the second evaluation, MgO-based additive was added to ES to obtain ES-A (ES with 0.05 wt% MgO-based additive) and ES-B (ES with 0.10 wt% MgO-based additive). DTF combustion test for ES-A and ES-B shows that the addition of MgO can reduce the risk of ash deposit risk, indicated by lower ash deposition rate and fewer melting spots on the probe surface. However, the presence of Na is still found in ash particles of ES-B, so dosage of 0.10 wt% MgO-based additive is more promising in mitigating ash deposition risk of ES. It is worth noting that this test is conducted in a laboratory scale which pays attention to test parameters according to real conditions. Therefore, further testing in a pilot-scale may be recommended before these findings are implemented in a power plant.